1. Field of the Invention
The present invention relates to a lithographic apparatus, a device manufacturing method and a device manufactured thereby. In particular the invention concerns critical dimension (CD) profile correction in lithography.
2. Background to the Invention
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist) on it. In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
One problem encountered in lithography is that the critical dimension (CD), namely the width of a patterned line that can be printed (on the wafer) within design tolerances, varies across the wafer. Commonly the linewidth that can be printed within set design tolerances varies across the wafer, either increasing steeply or decreasing steeply at the edge of the wafer. We refer to this phenomenon as variation in the CD profile across the wafer and the pattern of this variation as the “CD fingerprint” of the wafer. This is caused by downstream process steps (e.g., etch, annealing) and commonly has the effect that, for a set of lines of uniform width to be printed on the wafer, the actual linewidth which is printed decreases or increases towards the edge of the wafer (whether the linewidth decreases or increases usually this depends on whether negative or positive resist is used on the wafer in the lithographic process). This is illustrated in FIG. 1 (which is a plan view of the critical dimension profile across a wafer) by the change in tonal shading towards the edge of the wafer. This results in a decrease in yield (i.e., less useable chips from each printed wafer) for the chip manufacturer.
One known technique to address this problem involves trying to compensate for the variation in CD profile at the edge of the wafer by using predetermined offsets in the dose (i.e., amount of exposure) applied to particular areas or “fields” on the wafer, usually to individual die at or near the edge of the wafer, when these fields are exposed with the pattern to be printed onto them. For example certain fields may be exposed with higher or lower energy doses of the patterned light being imaged onto the wafer (than would be used where no such “CD correction” technique is employed). However this has the disadvantage that the whole field has an offset applied to it. This results in an over-correction on one side of the field and under-correction on the other side.
Another problem issue is that in some cases the CD profile may in some cases be relatively flat across the wafer, but one or more device performance characteristics may vary in a plurality of devices formed at different positions across the wafer (where the plurality of devices were all designed to have the same said device performance characteristics).